1. Field of the Invention
The invention relates to a method of generating X-ray images by means of an X-ray image converter and a CCD image sensor.
2. Description of the Related Art
Such a method and X-ray apparatus are known from U.S. Pat. No. 4,901,336. They are based on an image sensor which converts the output image of an X-ray image intensifier into electric signal and which is conceived for a given image frequency (30 frames per second). At the end of each image period, the image is transferred, using a transfer pulse, to the storage section of the CCD image sensor wherefrom it is read out during the generating of the next image. In order to enable the such an image sensor to form also X-ray images whose exposure time exceeds one image period the series of transfer pulses is interrupted during an X-ray exposure.
In the case of an angiographic examination, for example an examination of the coronary vessels by means of a contrast medium, however, the aim is to obtain an X-ray image in an as short as possible period of time. Such an examination first takes place in a fluoroscopy made during which the examination zone is continuously irradiated with a comparatively low dose rate. The fluoroscopic images have a small signal-to-noise ratio due to the low dose administered or the small dose per fluoroscopic image. The image quality thus obtained, however, is sufficient to trace the propagation of the contrast medium in the vessels.
In order to enable reproduction of given phases of propagation of the contrast medium, X-ray images are made. These X-ray exposures are repeated with a time interval which is long in comparison with the duration of an image period (in the fluoroscopy mode). In order to obtain a suitable signal-to-noise ratio, each X-ray exposure requires a dose which is substantially higher than these of a single image in the fluoroscopy mode, and the exposure duration should be shorter than the duration of one image period in the fluoroscopy mode in order to ensure that the motional unsharpness in the X-ray image remains limited.
However, it has been found that such CCD image sensor is not capable of achieving the image quality that can be achieved by means of a directly exposed X-ray-film intensifier foil system with such high doses and high spatial frequencies.
It is an object of the present invention to conceive a method of the kind set forth so that an improved image quality of the X-ray image is achieved, also in the case of fast physiological processes. This object is achieved in accordance with the invention in that the method comprises the following steps:
a) Performing a first X-ray image exposure during a first time interval which is shorter than the read-out period of an image stored in the storage section of the CCD image sensor,
b) transferring the first image generated in the image section of the CCD image sensor during the first X-ray exposure to the storage section thereof after expiration of the first time interval,
c) performing a second X-ray exposure directly after or at a short distance in time from the first X-ray exposure during a second time interval which is short in comparison with the read-out period,
d) reading out the first image from the storage section and writing the image into a further memory,
e) transferring the second image to the storage section after the reading out of the first image,
f) reading out the second image and summing it with the first image stored in the further memory so as to produce the X-ray image.
The invention is based on the following considerations:
In the case of a small X-ray dose per single image, the signal-to-noise ratio of this single image is determined essentially by the statistics of the X-ray quanta, because the noise caused by the transfer chain consisting of the X-ray image converter and the CCD sensor is negligibly small in comparison with the quantum noise.
In the case of a high X-ray dose per single image, however, the limited storage capacity of the CCD image sensor becomes manifest: customary CCD image sensors can only store a given number of electrons per CCD cell (a CCD cell stores a charge corresponding to the brightness at a pixel). For video sensors, this number amounts to from 50,000 to 100,000 electrons. In order to avoid overdriving of the CCD image sensor, the mean image brightness should correspond to a lower number, for example 20,000. This corresponds to a white noise (shot noise) of 141 electrons. This noise is independent of the spatial frequency.
However, because of the pronounced decrease of the modulation transfer function for high spatial frequencies, the brightness signal (and hence the associated noise determined by the statistics of the X-ray quanta) is reduced to such an extent that the noise is no longer determined by the statistics of the X-ray quanta, but rather by the statistics of the charge carriers that can be stored in a CCD cell. The spatial resolution of the detector is thus limited.
In accordance with the invention, the image quality is improved in that (at least) two images are added so as to form an X-ray image so that the number of electrons per pixel is increased accordingly by a factor 2. As a result, the signal-to-noise ratio determined by the statistic of the charge carriers that can be stored in a CCD cell is increased in conformity with the square root of this factor (3 dB).
From U.S. Pat. No. 4,901,336 it is known per se to add a plurality of single images generated during an image period so as to obtain an X-ray image. Therefore, the exposure time of such an X-ray image amounts to a multiple of a single image period (40 ms). Therefore, the known method is suitable only for slowly moving or even stationary structures. This known method is not suitable for the recording of fast motions, for example the propagation of a contrast medium in the coronary vessels.